Conventional technologies of this sort include the hydraulic drive system disclosed in JP-A-2001-105954. This conventional technology is equipped with a pair of hoist cylinders for vertically pivoting a body, in other words, a load carrying platform, a hydraulic pump for feeding pressure oil that operates these hoist cylinders, and a hoist valve, specifically a directional control valve arranged between the hydraulic pump and the hoist cylinders to control an operation of the hoist cylinders.
Only one directional control valve is arranged as the directional control valve. This directional control valve has four switch positions, which are a switch position for performing a platform raising operation to pivot the load carrying platform in an upward direction, a switch position for performing a platform forced-lowering operation to forcedly pivot the load carrying platform in a downward direction by pressure oil delivered from the hydraulic pump, a switch position for performing a platform free-fall permitting operation to pivot the load carrying platform in a downward direction by its own weight, and a switch position for performing a platform stay holding operation to enable the holding of an aerial stay of the load carrying platform, in other words, a neutral position. It is to be noted that the switch position for performing the above-mentioned platform free-fall permitting operation is formed between the neutral position and the switch position for performing the above-mentioned platform forced-lowering operation.
The conventional technology is constructed to also include, as a switching control means capable of outputting a pilot pressure to switch this directional control valve, a solenoid proportional pressure control valve controllable in opening area responsive to an analog signal outputted from a controller and connected to an auxiliary hydraulic pump, i.e., a pilot pump and a directional solenoid valve switchable to one of two switch positions responsive to a digital signal outputted from the controller to feed a pilot pressure, which has been outputted from the solenoid proportional pressure control valve, to one of left and right control ports of the above-mentioned directional control valve.
In this conventional technology, the solenoid proportional pressure control valve is controlled by the analog signal outputted from the controller, and the directional solenoid valve is switched by the digital signal outputted from the controller. By the pilot pressure outputted from the directional solenoid valve, the directional control valve is switched to one of the above-mentioned switch position for performing the platform raising operation, the above-mentioned switch position for performing the platform forced-lowering operation and the above-mentioned switch position for performing the platform free-fall permitting operation, whereby the pressure oil delivered from the hydraulic pump is fed to the hoist cylinder via the directional control valve to perform the corresponding operation.
When the output of the pilot pressure from the solenoid proportional pressure control valve is stopped by control from the controller, the directional control valve returns to the neutral position to perform the above-mentioned platform stay holding operation.